This invention relates to methods and apparatus for coding digital information and more particularly to such methods and apparatus for use in digital audio broadcasting systems
Digital Audio Broadcasting (DAB) is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM In-Band On-Channel (IBOC) DAB signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog signal, or in an all-digital format without the analog signal. IBOC DAB requires no new spectral allocations because the digitally modulated signal and the analog signal are simultaneously transmitted within the spectral mask of an existing channel allocation. IBOC DAB promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
An orthogonal frequency division multiplex (OFDM) technique has been described for IBOC DAB. OFDM signals include orthogonally spaced carriers modulated at a common symbol rate. The frequency spacing for symbol pulses (e.g., BPSK, QPSK, 8 PSK or QAM) is proportional to the symbol rate. For hybrid IBOC transmission of AM compatible DAB signals, sets of OFDM sub-carriers are placed within about 5 kHz to 15 kHz on either side of a coexisting analog AM carrier, and additional OFDM sub-carriers are placed within a ±5 kHz frequency band occupied by the analog modulated AM carrier.
DAB systems utilize forward error correction (FEC) and interleaving to improve the reliability of the transmitted digital information over corrupted channels. Most conventional convolutional codes have been designed to perform well with binary signaling in an additive white noise Gaussian (AWGN) channel. The simplest codes have rate of 1/n, where each input information bit produces n output bits. Punctured codes can be constructed by removing code bits from a rate 1/N “mother code” to produce a higher rate code. S. Kallel, “Complementary Punctured Convolutional (CPC) Codes and Their Applications,” IEEE Trans. Comm., Vol. 43, No. 6, pp. 2005-2009, June 1995, described a technique for producing complementary codes, which employs a sort of puncturing technique to create good component codes.
B. Kroeger, D. Cammarata, “Robust Modem and Coding Techniques for FM Hybrid IBOC DAB,” IEEE Trans. on Broadcasting, Vol. 43, No. 4, pp. 412-420, December 1997 described a technique to create overlapping component codes without all of Kallel's requirements on the complementary property. In U.S. patent application Ser. No. 09/438,822 (WIPO International Publication No. WO 01/35555), Kroeger et al. have also shown that these codes can be mapped onto QAM symbols using a Pragmatic Trellis Code Modulation (PTCM) technique described by Viterbi et al., in “A Pragmatic Approach to Trellis-Coded Modulation”, A. Viterbi et al. IEEE Communications Magazine, pp. 11-19, Vol. 27, No. 7, July 1989, while also preserving the complementary-like properties.
The free distance (dfree) of a convolutional code (punctured or non-punctured) is a convenient metric to gauge error correction performance in an AWGN channel with binary signaling (e.g. BPSK or QPSK). Secondary metrics such as the number of paths at the free distance, and the number of errors on those paths are used to resolve finer performance differences. The Optimum Distance Profile is also useful, especially for codes with large constraint length. When nonbinary signaling is used, such as QAM in an AWGN channel, the minimum Euclidean distance metric through the trellis paths is significantly more appropriate. Unfortunately trellis code modulation (TCM) and PTCM were designed for AWGN channels and do not perform well in impulsive noise. This is because the PTCM (or TCM) codes provide no error protection on the most significant bits with the larger uncoded Euclidean distances in the QAM constellation. Hamming distance is more important for error protection in an impulsive noise channel.
There is a need for a coding technique that overcomes these limitations and is suitable for use in IBOC DAB systems.